1. Field of the Invention
This invention relates to a shift mechanism for an outboard motor.
2. Description of the Related Art
In outboard motor shift mechanisms, a shift is usually changed by moving a shift rod having a cam at its distal end in the longitudinal direction to slide a shift slider such that a clutch is switched from its neutral position to a forward position where it engages with a forward gear or a reverse position where it engages with a reverse gear.
Alternatively, a shift rod is provided with a rod pin at a position eccentric from the rod center, in such a way that a shift slider is slid to effect shift by a distance due to a displacement of the rod pin caused by a rotation of the shift rod. The distance of travel of the rod pin corresponds to a circular arc whose radius is the amount of eccentricity of the rod pin.
In the outboard motor shift mechanisms including that mentioned above, when the shift rod is operated manually, since the operator tends to have an unpleasant operation “feel” owing to, for instance, heavy load, it has hitherto been proposed installing an actuator at the hull and connecting it with the shift rod in the outboard motor through a cable or a link mechanism to power-assist the driving of the shift rod, i.e. the shift, as taught in Japanese Laid-Open Patent Application No. Hei 4 (1992)-95598.
The add-on system using such an actuator has disadvantages that its structure is complicated, that it adds to the number and weight of the components, and it needs a space for the actuator at the hull.
Further, the outboard motor shift mechanisms are usually constituted as a meshed type of clutch comprising a shifter clutch and forward/reverse gears to be meshed therewith, i.e., the so-called “dog clutch”. In this type of clutch, unless the rotation of drive shaft side (forward/reverse gears) and that of driven shaft side (shifter clutch) are in synchronism with each other, projections formed thereon do not fit into mated recesses smoothly at the beginning of shift, thereby causing shock to happen. With this, the drive train (including the drive shaft, propeller shaft) may have excessive stress.
In order to avoid this problem, it has been known to mitigate such an excessive stress by dividing the drive shaft and propeller shaft into two parts and by connecting them through an elastic member, as disclosed in Japanese Laid-Open Patent Application No. 2000-280983. However, this also has disadvantages that its structure is complicated, that it adds to the number and weight of the components.
Furthermore, when an actuator is installed to move the shift rod, the shift rod may preferably be moved without using the actuator, in case of failure of the actuator.